[Xen-changelog] [xen-unstable] Do not spin on locks that may be held by stop_machine_run() callers.

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1269253753 0
# Node ID 4d3834df04831658eee8f2b7398443e24d4eb2fe
# Parent  81d9132ce00d353c5a9d6850bfd902304f65b54a
Do not spin on locks that may be held by stop_machine_run() callers.

Currently stop_machine_run() will try to bring all CPUs to softirq
context, with some locks held, like xenpf_lock or cpu_add_remove_lock
etc. However, if another CPU is trying to get these locks, it may
cause deadlock.

This patch replace all such spin_lock with spin_trylock. For
xenpf_lock and sysctl_lock, we try to use hypercall_continuation, so
that we will not cause trouble to user space tools. For
cpu_hot_remove_lock, we simply return EBUSY if failure, since it will
only impact small number of user space tools.

In the end, we should try to make the stop_machine_run as spinlock
free.

Signed-off-by: Jiang, Yunhong <yunhong.jiang@xxxxxxxxx>
---
 xen/arch/x86/platform_hypercall.c |   13 +++++++++++--
 xen/arch/x86/smpboot.c            |   20 +++++++++++++++++---
 xen/common/sysctl.c               |    6 +++++-
 3 files changed, 33 insertions(+), 6 deletions(-)

diff -r 81d9132ce00d -r 4d3834df0483 xen/arch/x86/platform_hypercall.c
--- a/xen/arch/x86/platform_hypercall.c Mon Mar 22 10:24:17 2010 +0000
+++ b/xen/arch/x86/platform_hypercall.c Mon Mar 22 10:29:13 2010 +0000
@@ -73,7 +73,11 @@ ret_t do_platform_op(XEN_GUEST_HANDLE(xe
     if ( op->interface_version != XENPF_INTERFACE_VERSION )
         return -EACCES;
 
-    spin_lock(&xenpf_lock);
+    /* spin_trylock() avoids deadlock with stop_machine_run(). */
+    while ( !spin_trylock(&xenpf_lock) )
+        if ( hypercall_preempt_check() )
+            return hypercall_create_continuation(
+                __HYPERVISOR_platform_op, "h", u_xenpf_op);
 
     switch ( op->cmd )
     {
@@ -398,7 +402,12 @@ ret_t do_platform_op(XEN_GUEST_HANDLE(xe
 
         g_info = &op->u.pcpu_info;
 
-        spin_lock(&cpu_add_remove_lock);
+        /* spin_trylock() avoids deadlock with stop_machine_run(). */
+        if ( !spin_trylock(&cpu_add_remove_lock) )
+        {
+            ret = -EBUSY;
+            break;
+        }
 
         if ( (g_info->xen_cpuid >= NR_CPUS) ||
              (g_info->xen_cpuid < 0) ||
diff -r 81d9132ce00d -r 4d3834df0483 xen/arch/x86/smpboot.c
--- a/xen/arch/x86/smpboot.c    Mon Mar 22 10:24:17 2010 +0000
+++ b/xen/arch/x86/smpboot.c    Mon Mar 22 10:29:13 2010 +0000
@@ -1342,7 +1342,12 @@ int cpu_down(unsigned int cpu)
 {
        int err = 0;
 
-       spin_lock(&cpu_add_remove_lock);
+       /* spin_trylock() avoids deadlock with stop_machine_run(). */
+       if (!spin_trylock(&cpu_add_remove_lock)) {
+               err = -EBUSY;
+               goto out;
+       }
+
        if (num_online_cpus() == 1) {
                err = -EBUSY;
                goto out;
@@ -1384,7 +1389,10 @@ int cpu_up(unsigned int cpu)
 {
        int err = 0;
 
-       spin_lock(&cpu_add_remove_lock);
+       /* spin_trylock() avoids deadlock with stop_machine_run(). */
+       if (!spin_trylock(&cpu_add_remove_lock))
+           return -EBUSY;
+
        if (cpu_online(cpu)) {
                printk("Bring up a online cpu. Bogus!\n");
                err = -EBUSY;
@@ -1419,6 +1427,8 @@ void disable_nonboot_cpus(void)
                if (cpu == 0)
                        continue;
                error = cpu_down(cpu);
+               /* No need to check EBUSY here */
+               ASSERT(error != -EBUSY);
                if (!error) {
                        cpu_set(cpu, frozen_cpus);
                        printk("CPU%d is down\n", cpu);
@@ -1439,6 +1449,8 @@ void enable_nonboot_cpus(void)
        mtrr_aps_sync_begin();
        for_each_cpu_mask(cpu, frozen_cpus) {
                error = cpu_up(cpu);
+               /* No conflict will happen here */
+               ASSERT(error != -EBUSY);
                if (!error) {
                        printk("CPU%d is up\n", cpu);
                        continue;
@@ -1481,7 +1493,9 @@ int cpu_add(uint32_t apic_id, uint32_t a
        if ( physid_isset(apic_id, phys_cpu_present_map) )
                return -EEXIST;
 
-       spin_lock(&cpu_add_remove_lock);
+       /* spin_trylock() avoids deadlock with stop_machine_run(). */
+       if (!spin_trylock(&cpu_add_remove_lock))
+               return -EBUSY;
 
        cpu = mp_register_lapic(apic_id, 1);
 
diff -r 81d9132ce00d -r 4d3834df0483 xen/common/sysctl.c
--- a/xen/common/sysctl.c       Mon Mar 22 10:24:17 2010 +0000
+++ b/xen/common/sysctl.c       Mon Mar 22 10:29:13 2010 +0000
@@ -48,7 +48,11 @@ long do_sysctl(XEN_GUEST_HANDLE(xen_sysc
     if ( op->interface_version != XEN_SYSCTL_INTERFACE_VERSION )
         return -EACCES;
 
-    spin_lock(&sysctl_lock);
+    /* spin_trylock() avoids deadlock with stop_machine_run(). */
+    while ( !spin_trylock(&sysctl_lock) )
+        if ( hypercall_preempt_check() )
+            return hypercall_create_continuation(
+                __HYPERVISOR_sysctl, "h", u_sysctl);
 
     switch ( op->cmd )
     {

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